The present invention relates to laser control systems, and more particularly, but not exclusively, to laser welding systems in which the power density is dynamically varied based on the beam's position on the parts to be welded.
One of the most difficult challenges to laser welding two materials together compensating for different melting points or thermal characteristics in the materials. The typical approach to welding materials with dissimilar melting points or of different thicknesses is to apply more energy to the material with the higher melting point. This has traditionally been done by moving the laser beam parallel to the weld junction with the center of the laser beam offset from the weld junction. Due to this offset, more of each laser spot lands on one side of the weld junction than the other, and thus a correspondingly greater proportion of the laser energy from each pulse is applied to the higher melting point material. However, having substantially unequal areas of each material being subject to each laser pulse typically leads to differing amounts of each material being melted into the weld pool, and a such an asymmetrical weld pool can compromise weld quality. Conversely, if the laser spot is more centered on the weld junction in an effort to melt more equal amounts of each material, there is a risk that the more fragile material would be obliterated or that there would be insufficient melting of the more durable material, which can also comprise the weld quality. Thus, selecting the proper offset that achieves the ideal distribution of energy between the two materials often must be determined through trial and error.
Since the development of nearly diffraction limited diode pumped solid state lasers and fiber lasers, it has become possible to focus the laser energy onto a greatly reduced spot size, for example, to a spot diameter about 1/10 the size of the spot of a conventional Nd:YAG laser. This small laser spot size has made welding along a seam more challenging. The primary difficulty lies in the need to apply a sufficient amount of energy to melt each material. Due to the greatly reduced spot size, the inherent gaps between the materials becomes more significant, and it becomes nearly impossible to apply sufficient energy to each material by simply offsetting a single spot relative to the material junction, as the small spot size is not large enough to encompass a sufficient amount of each material to create an adequate weld pool.
Thus, in order to assure that appropriate amounts of energy are applied to each material, a highly focused laser beam needs to be cycled back and forth across the junction, typically in a zig zag pattern. This movement can be accomplished by moving the part relative to a stationary laser beam, or more typically, by utilizing two-dimensional beam steering optics to steer the beam in a specified pattern across the material junction. Typical beam steering optics utilize mirrors and a two-axis galvanometer steering head to steer the beam in two dimensions.
By moving a highly focused laser beam along a zig zag path that traverses the material junction, the laser energy is applied over a wider area of each material than could be accomplished by simply moving the beam parallel to the junction. However, for the same reasons the larger diameter spots of conventional Nd:YAG lasers were offset from the material junction, the zig zag pattern is also typically offset relative to the material junction such that the laser energy is applied over a greater area of the higher melting point material than of the lower melting point material, which can likewise yield an asymmetrical weld pool due to dissimilar amounts of each material being melted.
Furthermore, as the laser moves, it is typically operated so as to produce either a continuous beam or a series of short duration pulses at a specified pulse repetition rate. When the laser is operating in a pulsed mode at low to mid-frequencies up to about 5 kHz, which many precision fiber laser welding applications require, the relationship between the frequency of the zig-zag pattern and the pulse repetition rate becomes important. For example, if the zig-zag motion is operating at a frequency of 1 kHz and the pulse rate of the laser is also 1 kHz, the laser pulses will not be distributed across the desired welding zone but will instead occur at the same relative location during each pass, for example, with all pulses being in line with the material junction or all on one side or the other, depending on the point of the cycle at which the pulse train of the laser commences. Therefore, for many precision welding applications, it is typically necessary to have the pulse rate of the fiber laser be significantly greater than the period of the movement across the junction, which limits the overall welding speed for the application.
Accordingly, there is a need for improvements in this area.